This application incorporates by reference Taiwanese application Serial No. 88116138, filed Sep. 17, 2000.
1. Field of the Invention
The invention relates in general to an image preprocessing apparatus of an image capturing apparatus, and more particularly to an image preprocessing apparatus of an image capturing apparatus which employs multiplexers to deal with analog signals obtained from black-and-white (B/W) scanning and color scanning.
2. Description of the Related Art
Referring now to FIG. 1, a conventional image capturing apparatus is shown in a block diagram form. The image capturing apparatus includes an image sensing module 10, an analog signal processor 12, and an image processing circuit 14. The image sensing module 10 scans the objects, such as documents or photos, and outputs analog signals of the resulting image. The image sensing module 10 includes three sensors for detecting color images. Outputs of the three sensors are analog signals respectively corresponding to red (R), green (G), and blue (B) component images of an image of a scanned object. Since the signal levels of these analog signals are different, these analog signals are processed and converted to digital signals by an analog signal processor 12. Finally, the output digital signals are transmitted to an image processing circuit 14 for further image processing.
The image sensing module 10 for color scanning generally fall into two categories: charge coupled device (CCD) modules and contact image sensor (CIS) modules. In the following, the CCD module is discussed first.
Since visible light is composed of three primary colors, namely, red (R), green (G) and blue (B), the color CCD module generally includes image sensors corresponding to these three colors. Referring to FIG. 2, structure of a CCD module is shown in block diagram form. The CCD module 20 includes three monochrome sensors including sensor 22, 24, and 26, which respectively detect the R, G, and B components of the light reflected by the scanned object. In general, many documents are not colored and so allowances for B/W scanning must also be provided for in the CCD module. Currently, there are three techniques for B/W scanning. The first technique involves using one of the monochrome sensors 22, 24, or 26, originally employed in a CCD module, as the sensor for B/W scanning. Sensor 24 for detecting the green component (G-sensor) is generally taken for B/W scanning. However, there is a drawback in practice. When the G-sensor 24 is used for B/W scanning, the complementary color of green will be filtered out (i.e. red will vanish) resulting in a distortion of the scanned image. The second technique involves photosensitizing the three monochrome sensors 22, 24, and 26 simultaneously and then combining the output R, G, and B component image signals by image processing circuits or software computation to produce B/W images. The disadvantage of this technique is that the processing rate is lowered. The third technique involves providing an additional B/W CCD image sensor 28 in the CCD module 20, as shown in FIG. 2.
Currently there are two structurally differentiated CCD image sensors. Referring now to FIG. 3, the structural diagram of a monochrome CCD image sensor with one shift register, which is called a single monochrome CCD image sensor, is shown. The single monochrome CCD image sensor 30 consists of a shift register 32, which is a CCD, as well as a photosensitive device 34. The shift register 32 is so named because during a clock cycle, it is capable of shifting the image data of a pixel to the next stage of the register in one direction only.
Another structure of a monochrome CCD image sensor is one having two shift registers and thus appropriately called a dual monochrome CCD image sensor. As shown in FIG. 4, because the dual monochrome CCD image sensor 40 has dual shift registers to process the image data, the data transfer rate of this sensor 40 is double that of the sensor 30 assuming that these shift registers are operating at the same clock frequency. In addition, the dual monochrome CCD image sensor 40 provides two outputs of image data, namely, the odd pixel output and even pixel output. For the single monochrome CCD image sensor 30 only one output of image data is provided.
As discussed above, in the implementation of the third technique, a B/W CCD image sensor is included in the image sensing module. Thus, an additional circuit for processing the analog output from the B/W CCD image sensor is generally required. This also leads to an additional analog signal path. Since analog signals are not as capable of bearing the effect of crosstalk as digital signals, the crosstalk due to this analog signal path would cause degradation of signal quality.
The primary objective of the invention is to provide an image preprocessing apparatus using multiplexers to process analog signals obtained from B/W and color scanning. By the invention, the analog signals obtained from B/W and color scanning are processed by the same analog signal processor, resulting in a simplification of circuit design, decreased production costs, as well as decreased crosstalk effects.
In accordance with the objective of the invention, an image preprocessing apparatus, which processes image signals of an image of a scanned object, includes an image sensing module, a multiplexer, and an analog signal processor. The image sensing module includes a number of monochrome image sensors and a B/W image sensor, all of which output analog signals. The monochrome sensors include individual sensors for detecting red, green, and blue, which output analog signals of red, green, and blue component images of the image of the scanned object respectively. The multiplexer is for receiving the B/W analog signal and some monochrome analog signals and for selecting either the B/W analog signal or these monochrome analog image signals as the output of the multiplexer. The analog signal processor is for receiving the output analog signal of the multiplexer and other monochrome analog image signals that are not received by the multiplexer, and for outputting a digital signal.
In accordance with the objective of the invention, an image sensing module includes a number of monochrome image sensors, a B/W image sensor, and a multiplexer. The monochrome image sensors are for outputting a number of monochrome analog image signals each of that corresponds to specific color. The monochrome image sensors include individual sensors for detecting red, green, and blue, and outputting analog signals of red, green, and blue respectively. The B/W image sensor is for outputting a B/W analog image signal. The multiplexer is for receiving the B/W analog signal and some monochrome analog signals and for selecting either the B/W analog signal or these monochrome analog image signals as the output of the multiplexer.